说明：  stm32 f0xx f2xx f3xx f4xx 系列RTC的使用说明。对RTC的使用非常有用。
(A real-time clock (RTC) is a computer clock that keeps track of the current time. Although RTCs are often used in personal computers, servers and embedded systems, they are also present in almost any electronic device that requires accurate time keeping. Microcontrollers supporting RTC can be used for chronometers, alarm clocks, watches, small electronic agendas, and many other devices.)